Signal blocking and fading in mobile communications systems is well known. Satellite-based systems generally have more stringent requirements that terrestrial-based systems due to the significantly longer propagation paths. In satellite mobile communications systems blocking and fading of user terminals by buildings, trees and terrain can be mitigated by using multiple orbiting satellite repeater transmitters to send multiple copies of a signal, via some or all satellite repeater transmitters in view, to a user which is potentially experiencing signal blocking and fading. These mitigation techniques, especially those using spread spectrum systems, utilize multiple signal path diversity (hereinafter referred to simply as xe2x80x9cpath diversityxe2x80x9d) as a means of maintaining communication paths when individual mobile users are in blocking and fading situations. Low Earth Orbit (LEO) satellite communication systems in particular can exploit path diversity since there are multiple satellites and, hence, multiple and different communication paths to and from the user.
Most known or proposed systems of this type, in addition to using Code Division Multiple Access (CDMA), generally channelize by frequency division multiplex (FDM). Furthermore, providing path diversity has an adverse affect of requiring the system to utilize many satellites. This increases the total power demand for each satellite, and also requires each satellite to make the same RF channels available for each user for path diversity transmissions. The end result can be a reduction in the total capacity of the system due to inefficiencies in RF channel assignment.
One approach to providing path diversity is to provide path diversity indiscriminately to all users. In fact, however, the inventor has realized that there are many different types of user terminals, as well as many different types of communication environments that a given user may reside in, either temporarily or permanently. For example, certain users will employ vehicle mounted terminals which can move through the environment rather quickly. Other users may employ hand-held or fixed terminals which may not be moving at all. In addition, there are a variety of terrains wherein users can be located, such as oceans, deserts, forests, suburban, urban, rural farmland, etc.
It can be appreciated that not all communication environments require the same level of path diversity, and furthermore not all user terminals within a given environment require the same level of path diversity.
The foregoing and other problems are overcome by a satellite communications system that is constructed and operated in accordance with this invention.
Methods and apparatus are disclosed for improving and optimizing the delivery of path diversity in a satellite repeater-based communication system, thereby conserving both FDM channels and satellite power utilization. The reception of multiple signals is improved, when one or more orbiting satellite repeater (12) transmitters is blocked or severely faded, by recognizing the need for satellite path diversity on a real-time or near real-time basis. Thus, a user terminal (13) is enabled to receive sufficient signal strength to avoid having an ongoing communication automatically terminated by the optimization of path diversity (formed by multiple wireless links) that is applied to (a) classes (types) of user terminals and/or (b) to individual user terminals as a function of location and also a local RF propagation environment of the user terminal. In addition, the invention teaches a consideration of the satellite resources that are available at any given point in time, and may restrict or limit the availability of satellite path diversity, thereby increasing overall system capacity.
Also, a particular user may be found to have a historical record or xe2x80x9csignaturexe2x80x9d of operating within a certain environment. The historical record can be utilized to optimize the user""s typical usage, thereby further refining the potential for gaining higher system operating efficiency.
This invention teaches a method for operating a satellite communication system that includes the steps of (a) initiating a communication between a user terminal and a ground station via at least one satellite communication signal repeater; (b) classifying the user terminal as to type and/or determining a location of the user terminal within a service coverage area of the ground station; and (c) selecting a number of satellite communication signal repeaters to relay the communication between the user terminal and the ground station, the selected number being a function of at least the type and/or location of the user terminal and other characteristics, which may be stored within a database. The step of selecting can include a step of determining an RF energy propagation characteristic that is associated with the determined location of the user terminal. The use of a service area RF propagation map is disclosed for this purpose, the map being derived from, by example, satellite images of the natural and man-made features within the service area. The step of selecting can also include a step of considering a power control history of the user terminal. This is useful in distinguishing, by example, a mobile-type user terminal that is in motion from a mobile-type user terminal that happens to be stationary. The step of selecting may also include a step of considering a current availability of satellite communication signal RF channels within satellite repeaters and the physical circuit loading of the RF channels and satellite repeaters.
In a presently preferred embodiment of this invention the communication is relayed as a spread spectrum, code division multiple access communication signal between the user terminal and the ground station. In this case the method includes the additional steps of (d) receiving the communication with the user terminal, wherein the communication is received through different communication paths associated with individual ones of the selected number of satellite communication signal repeaters; (e) equalizing at least the phase shifts and time delays of the received communication from each of the different paths to provide a plurality of equalized communication signals; and (f) combining the equalized communication signals into a composite received communication signal.